The desert that feeds a forest: how Sahara dust carries phosphorus to the Amazon
NASA satellites measured dust leaving the Sahara, crossing the Atlantic and falling on the Amazon. The phosphorus in that dust, about 22,000 tons a year, is roughly what the rainforest loses to rain and flooding.
Key points
- About 182 million tons of dust a year are carried past the western edge of the Sahara.
- About 27.7 million tons of that dust falls on the Amazon basin.
- The dust brings an estimated 22,000 tons of phosphorus a year - about what the Amazon loses to rain and flooding.
- Much of the phosphorus-rich dust comes from the Bodele Depression in Chad, an ancient lake bed.
What happened
The Amazon rainforest looks rich, but its soils are poor. Rain washes nutrients out of them year after year, and phosphorus - which plants need to grow - is one of the nutrients that leaks away into streams and rivers. The puzzle was where the replacement phosphorus comes from.
Part of the answer flies in from another continent. Winds lift dust from the Sahara, carry it high across the Atlantic, and drop it over South America. In 2015, scientists using a NASA satellite published the first three-dimensional measurement of how much dust makes that journey, and how much phosphorus travels with it.
Claim by claim
Why this label: From NASA's summary of the study by Yu and colleagues, published in Geophysical Research Letters in 2015.
On the ladder: a claim earns Verified from primary evidence, or from two independent sources that do not copy each other.
Why this label: NASA adds that 132 million tons remain in the air near South America's eastern coast, and about 43 million tons travel on to settle over the Caribbean Sea.
On the ladder: a claim earns Verified from primary evidence, or from two independent sources that do not copy each other.
Why this label: This is lead author Hongbin Yu's estimate, as reported by NASA. It is an estimate from satellite data, not a direct weighing.
On the ladder: a claim earns Verified from primary evidence, or from two independent sources that do not copy each other.
Why this label: CALIPSO measured the dust in three dimensions, which is what made the first satellite-based phosphorus estimate possible.
On the ladder: a claim earns Verified from primary evidence, or from two independent sources that do not copy each other.
Why this label: NASA describes the phosphorus as a remnant in Saharan sands from the desert's ancient past as a lake bed.
On the ladder: a claim earns Verified from primary evidence, or from two independent sources that do not copy each other.
What we know / what we don't know yet
What we know
- Dust crossing from Africa to the Amazon is large, regular and measurable from space.
- The phosphorus it delivers is on the same scale as the phosphorus the Amazon loses to water.
- Amazon soils are short of nutrients, and fallen leaves alone do not replace everything that rain washes away.
What we don't know yet
- Exactly how much dust the Amazon needs to stay healthy - the researchers themselves left this open.
- How the flow will change in future as winds, rain and the Sahara itself change.
Why it matters
It is a reminder that Africa's environment does not stop at Africa's shores. Dust from a dry lake bed in Chad helps keep the world's largest rainforest fed, which means changes in one place can quietly affect life on the other side of an ocean.
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